This invention relates to an active or adaptive hybrid transformer circuit (will be called an active hybrid transformer circuit hereinafter in the instant specification) for use in bidirectional metallic cable communication to separate a transmission or transmit (TX) signal and a reception or receive (RX) signal.
A recent interest has been directed to high speed bi-directional communication trough metallic cables, such as 10 GBASE-T, among a plurality of terminal stations each of which has a transmitter/receiver connected through the metallic cables operable as a transmission path circuit. In this event, each terminal station must separate a transmit (TX) signal and a receive (RX) signal from each other so as to carry out the bi-directional communication. To this end, a hybrid transformer circuit is built in each terminal station.
Conventionally, Japanese Unexamined Patent Publication No. Hei 4-54718 (namely, 54718/1992) discloses a combination of a hybrid transformer circuit and an echo canceller. This structure makes it possible to accomplish high separation between the TX and the RX signals. However, the disclosed combination is complicated in structure and results in an increase of a size and is also not suitable for high speed communication.
Alternatively, Japanese Unexamined Patent Publication No. Hei 4-158637 (namely, 158637/1992) discloses an active hybrid transformer circuit which includes a voltage driver and a balancing network. Herein, the balancing network is formed by a variable hybrid circuit and a switch circuit for switching resistors and capacitors included in the variable hybrid circuit. Furthermore, it is to be noted that the voltage driver should be linearly operated.
Consideration would be made about application of such an active hybrid transformer circuit to a communication system of a high speed among an increased number of terminal stations. In this event, the voltage driver should have a high speed characteristic and a low non-linear distortion characteristic. However, it is practically difficult to realize such a voltage driver of a high speed and a low non-linear distortion characteristic as long as restrictions are practically imposed on a source voltage and power consumption.
Furthermore, disclosure is made about an active echo canceller in the article that is contributed to 2001 IEEE International Solid-State Circuit Conference, pages 310 to 311, and that is entitled “A CMOS Transceiver Analog Front-End for Gigabit Ethernet over CAT-5 Cables”. The active echo canceller mentioned in the article uses a replica transmit signal and serves to effectively remove a transmit signal from a receive signal. Specifically, the active echo canceller has a current drive transmitter which is formed by a digital-to-analog converter (DAC) and a main driver coupled to a transmission path circuit through a transformer. In addition, the active echo canceller further has a replica driver connected in parallel with the main driver.
With this structure, digital transmit codes are converted by the DAC into the transmit current or transmit signal. The transmit current is given through the main driver to the transformer and to the replica driver which produces the replica transmit signal representative of a replica of the transmit current. The receive signal is obtained by subtracting the replica transmit signal from the transmit signal by the use of an analog subtracter so as to cancel the transmit signal in the receive signal.
According to the inventor's experimental studies, it has been found out in an active echo canceller of the above-mentioned type that an amount of attenuation is restricted to at most 10 dB on account of a component returned from the transmit signal to the receive signal and that the analog subtracter with a low non-linear distortion can not be easy to really attain.